FIG. 1 illustrates a standard substrate cleaning system 10 known in the prior art. In the system 10, a nozzle 14 dispenses a fluid 18 onto a surface 22 of a rotating substrate 26, such as a semiconductor wafer. The fluid 18 spreads out over the surface 22 and fills a gap 30 underneath an acoustic transducer 34. The fluid in the gap is energized by acoustic energy from the transducer 34. The combined effect of the fluid 18 and the acoustic energy is to process or clean the surface, such as by the removal of small particles from the surface. U.S. Pat. No. 6,791,242 is representative of this type of prior art substrate cleaning system.
There are several problems with the prior art cleaning systems. First, dispensing fluid onto the surface 22 is wasteful because only fluid that fills the gap 30 is primarily involved in cleaning or processing the surface. Excess fluid on the surface can increase the cost associated with cleaning, which can be significant when specialty cleaning fluids are used. Second, excess fluid can create environmental and safety problems, such as when evaporation or atomization causes dangerous gasses to be released from the fluid.
Third, when the substrate 26 rotates slowly, the fluid can wrap around to the backside of the substrate, thereby contaminating the backside of the substrate as well as the substrate holding or mounting equipment. For this reason, higher rotation speeds are preferred during substrate processing. But higher rotation speeds mean that more process fluid must be dispensed on the surface in order to keep the gap filled with fluid, thereby creating more wasted fluid.
Fourth, when excess fluid sits on the surface, the properties and characteristics of the fluid, such as temperature and gas content, can change. For reasons such as these, a way to reduce the volume of fluid being dispensed during a substrate cleaning process is needed.